Whether driven or stationary, cutting tools used for operations such as drilling, milling, or reaming metal workpieces, remove unwanted material from a part by shearing it away from the parent material. This shearing action develops heat. As such, cutting tools frequently require coolant at the interface between the workpiece and the cutting surface of the cutting tool. Coolant reduces the heat generated by the machining operation by reducing the friction between the cutting tool and the workpiece and by transferring heat from the cutting tool and workpiece to the coolant. Such heat reduction is important, as excessive heat prematurely dulls the cutting tool, lowers the quality of cuts produced by the cutting tool, and shortens the useful lifespan of the cutting tool. As one of ordinary skill in the art would recognize, the coolant can be a variety of fluids, including water, water with an oil emulsion, oil, air, air entrained with oil, carbon dioxide, or other compounds.
Systems for providing coolant at the interface between cutting tools and their respective workpieces are known in the art. Some coolant systems supply coolant through the center of the cutting tool (“through-center coolant systems”), while others direct a stream of coolant at the interface between the cutter and the workpiece. For example, U.S. Pat. No. 5,419,661 discloses a method for providing coolant through the center of a driven tool assembly while at the same time keeping the coolant separate from the oil or grease used to lubricate and cool the gears and bearings within the driven tool assembly housing. Usually, the through-center coolant systems require high pressure coolant for their operation, while coolant systems that externally direct coolant to the cutter/workpiece interface often operate at low pressures.
Driven tools have long been known and widely used in lathes, turning centers, machining centers and other machine tools to produce features on parts in addition to the rotationally symmetrical features normally associated with machining (or “turning”) on a lathe or other machine tool. A driven tool assembly cuts keyways, key seats, holes, and other features that may not be located on the rotational axes of the part. The tooling that produces these features contains powered (sometimes referred to as “live” or “driven”) cutters that can remove the material from the part whether or not the main lathe spindle axis is rotating. Although there are other means for operating these cutting tools, the turret on a lathe or machine tool is what commonly indexes and drives the driven tool assembly. The turret can store the cutting tools and index them into the cutting position.
Driven tool assemblies traditionally have an input shaft for receiving rotatable power from the machine tool or from an integrated power supply and an output shaft with a device for holding a cutting tool. A spindle is one example of a type of output shaft. A housing and bearings support the input and output shafts, and gears or other means for power transmission transmit power from the input shaft to the output shaft. The bearings may be rolling element devices (anti-friction bearings), sliding contact devices (plain bearings), or other types of friction reducing devices, including hydro-static bearings, in which the spindle rides on a film of fluid. Traditionally, oil or grease lubricate the driven tool's gears and bearings.
Conventional driven tool assembly design strives to isolate the oil or grease bearing lubricant from the cutting coolant using seals. It is not uncommon for a driven tool assembly to operate at 5000 rpm. At these high speeds, the driven tool assembly generates considerable heat within the bearings, limiting the amount of time the driven tool assembly may be operated before it is allowed to cool. Additionally, the seals between the rotating surfaces of the shafts and the stationary walls of the housing apply a substantial amount of frictional drag, increasing the power required to effectively turn the cutting tool and generating additional heat. If the operator exceeds the running time, then the driven tool assembly may overheat, causing the seals to fail, the lubricant to escape, and coolant to leak into the driven tool assembly. While coolants used in the past may have been oil based coolants, the vast majority of today's coolants are water based coolants containing 90-95% water. Because the gear lubricant and the coolant are often incompatible, the coolant will wash away the remaining gear lubricant, causing bearing failure. Even if the driven tool assembly is salvageable, it is expensive to rebuild the tool and make it serviceable again. What is needed is a driven tool assembly that uses the cutting tool coolant to also lubricate and cool the driven tool assembly. Ideally, the driven tool assembly may also be operated continuously without overheating, may have high duty life, and may use a low pressure coolant source.